Devices that generate images are ubiquitous in today's technology. These devices include inkjet ejecting devices, toner imaging devices, textile printing devices, circuit board printing devices, medical printing devices, monitors, cellular telephones, and digital cameras, to name a few. Throughout the life cycle of these devices, the image generating ability of the device requires evaluation and, if the images contain detectable errors, correction. Before such an imaging device leaves a manufacturing facility, the device should be calibrated to ensure that images are generated by the device without perceptible faults. As the device is used, the device and its environment may experience temperature instabilities, which may cause components of the device to expand and shift in relation to one another. As the device is used, the intrinsic performance of the device may change reversibly or irreversibly. Consequently, the imaging generating ability of such a device requires evaluation and adjustment to compensate for the changes experienced by the device during its life cycle. Sometimes these evaluations and adjustments are made at time or usage intervals, while at other times the adjustments are made during service calls made by trained technicians.
Drop placement in inkjet printers, including solid ink printers, may be imperfect from variations in drop direction, drop speed, drop travel distance, or the image substrate speed. These variations may produce objectionable artifacts in ink images. Additionally, components or subsystems of an imaging device experience aging during the operational life of the imaging device and the effects of this aging may produce variations that adversely affect image quality. Consequently, the ability to calibrate and adjust components and subsystems in an imaging system in both the manufacturing and operational environment is important.
In order to calibrate inkjet imaging systems, test patterns are printed and imaged. Typically, the test patterns are printed onto image substrates, such as media sheets, either directly or indirectly, and the substrate is imaged using a scanner or the like. The image is then analyzed to determine the actual position of the ink drops ejected to form the image and these positions are compared to internal representations of the test pattern to detect discrepancies between the actual positions of the drops and the intended positions of the pixels. This analysis is complicated by image noise and/or missing drops. For example, some imaging systems print the test pattern onto a rotating image member, the test pattern is then transferred to a media sheet, and the media sheet is imaged for analysis. The rotating image member may be an anodized aluminum drum. Scratches in the drum finish may obscure drops in the image and be a source of image noise. Missing jets are inkjets that either do not eject drops when they receive firing signals to eject ink or respond to those signals in an intermittent manner. Image analysis that is able to successfully identify drop positions in an image in the presence of image noise and/or missing drops is, therefore, desirable.